A typical elevator system comprises an elevator car and counterweight, each suspended on opposite ends of hoist ropes which are disposed in an elevator hoistway. The elevator system also includes at least two sets of guide rails extending the length of the elevator hoistway, with each set of guide rails being disposed on opposite sides of the hoistway. The guide rails guide a plurality of roller guides attached to the elevator car. The roller guides ensure a smooth ride by preventing any unevenness of the rails from being transmitted to the elevator car.
There are several factors that impact the quality of the elevator car ride. One such factor is the total length of the hoistway. Longer hoistways require a greater number of guide rail segments stacked within the hoistway and a greater number of joints between the guide rail segments. A greater number of guide rail segments result in greater total weight of the guide rails, and the resultant loading causes the rails to deflect. Also, the joints between the guide rails result in some discontinuity. Even slightly deflected rails and minimal discontinuity in joints cause the elevator car to vibrate and move laterally.
Another factor that adversely affects ride quality is an aerodynamic consideration. During vertical travel of an elevator car within the hoistway, aerodynamic pulses from the hoistway doors and an aerodynamic pulse from the counterweight cause lateral movement and vibration in the elevator car.
To minimize the adverse impact of rail imperfections and aerodynamics on the ride quality of the elevator car, a conventional roller guide assembly includes a suspension system and a damping system. The suspension system typically comprises a helical spring and a stop associated with each roller of the roller guide assembly to restore the roller to its original position after the roller has been deflected by imperfections in the guide rails.
It is desirable to have a relatively soft suspension system to isolate the elevator car from rail imperfections. The helical springs provide a soft ride quality while the relatively rigid stops prevent the elevator car from contacting the rails in case the helical springs become completely compressed. The quality of the ride is greatly reduced when the car contacts or rides the stop because the relatively rigid stop transmits rail imperfections to the elevator car. In addition, the transition from riding on the helical spring to riding on the stop subjects the elevator car to a small shock.
A problem arises when an elevator car is unbalanced, which can occur during normal usage. This unbalance partially compresses the helical springs, reducing the clearance between the elevator car and the stops. The movement of the elevator car, which produces a dynamic side-to-side motion, together with the already compressed springs, can result in the elevator car impacting and possibly riding on the stops. Since the stops are relatively rigid, they transmit any unevenness in the rails to the occupants of the elevator car.
Another problem with current suspensions is that helical springs provide very little damping of oscillations of an elevator car, so special damping devices are added. Existing damping systems comprise a hydraulic cylinder to reduce vibration. However, the hydraulic damping system increases the stiffness of the suspension system which is not desirable because of the resulting increase in guide rail excitations transmitted to the car, which in-turn increases vibrational response. Additionally, hydraulic damping systems require regular maintenance, sustain wear, and increase cost of the overall system.